USB: airprime: unlock mutex instead of trying to lock it again

The following patch fixes a [probable] copy & paste mistake in
airprime.c. Instead of unlocking an acquired mutex, the actual
code tries to lock it again.

Signed-off-by: Leonardo Chiquitto <lchiquitto@novell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>